The invention relates to a method for the production of carbon filaments having a crystalline graphitic structure and a morphology defined by a fishbone-like arrangement of the graphite layers along an axis of the filament.
Carbon fibers are forms of carbon which are known in the art and which have a diameter of, normally, from 5 to 15 micrometers. They are flexible, light in weight, thermostable, and to a large extent, chemically inert, and they are good thermal and electrical conductors. Such carbon fibers are produced by carbonizing an organic polymer yarn, for example, of rayon or polyacrylonitrile to form a fiber built up from a multitude of fibrils. The most important parameter to characterize carbon fibers is the preferred orientation, i.e., the extent to which carbon or graphite microcrystallites are oriented parallel to the fiber axis. The thermal and electrical conductivity as well as the intrinsic tensile strength and Young's modulus increase with increased preferred orientation.
Carbon fibers can be divided into two categories, low-modulus fibers having a Young's modulus below about 140 g Pa and high-performance fibers having a Young's modulus above about 170 g Pa and having a very high tensile strength.
It is also known to prepare high-performance carbon fibers from polymer fibers such as polyacrylonitrile fibers by carbonizing the fibers and subsequently subjecting the fibers, in an inert gaseous atmosphere, to an elevated temperature. To endow the fibers with the desired high mechanical strength, they must be subjected to a tensile stress at the very high recrystallization temperature. Partly as a result of the mechanical force applied, the graphite layers in the fiber have their planes oriented parallel to the axis of the fiber.
It is apparent that the procedures required for the production of carbon fibers by presently known methods is very expensive. As a result, it has up till now only been possible for carbon fibers to be used in applications where the required amount of fibers is relatively small and the price of the finished product is not critical. For large scale commercial applications, however, the cost of producing carbon fibers in large quantities is still prohibitive.
In a more recent development a new method of manufacture of graphite fibers has been disclosed in which the fibers are produced at a high yield by the pyrolysis of a hydrocarbon gas, U.S. Pat. No. 4,391,787 (G. G. Tibbetts). In this method, gaseous methane, or the like, is contacted with a chromium oxide film on one side of a thin hydrogen-permeable wall, while concurrently dissolving hydrogen into the opposite side of the permeable wall, and heating the wall and the gas to a temperature of from 925.degree. to 1075.degree. C. to form on the first wall surface graphite fibers from the carbon containing gas.